1. Field of the Invention
The present invention relates to a gas barrier material having superior gas barrier properties under conditions of high humidity, and possessing high clarity and melt-moldability, and also to a multilayer structure comprising a layer of said gas barrier material.
2. Description of the Background
Foods and medicines are currently packaged with a gas barrier material made of polymers having good gas barrier properties to isolate contents from oxygen or any other gas. Examples of such polymers are vinyl alcohol polymer typified by ethylene-vinyl alcohol copolymer (occasionally referred to as EVOH hereinafter), vinylidene chloride polymer (occasionally referred to as PVDC hereinafter), and acrylonitrile polymer (occasionally referred to as PAN).
EVOH exhibits good gas barrier properties under conditions of low humidity and is capable of melt-molding, especially coextrusion molding with polyolefin. Unfortunately, its gas barrier properties deteriorate under conditions of high humidity and hence it is limited in application and form in which it is used. This holds true for containers (such as juice containers) which need flavor retention. In this application, EVOH usually constitutes the innermost layer and hence it does not exhibit barrier properties satisfactorily. To compensate for this, it is necessary to add an intermediate EVOH layer. This adds to the production cost. Moreover, EVOH lacks moisture barrier characteristics. This makes it necessary to use, in many applications, EVOH in the form of laminate in combination with a material (such as polyethylene and polypropylene) having comparatively good moisture barrier properties. There still are instances where EVOH cannot be used because of its inability to meet stringent requirements for moisture barrier properties.
As compared with EVOH, PVDC is less dependent on humidity for gas barrier properties and hence exhibits good gas barrier properties even under conditions of high humidity. In addition, it possesses superior moisture barrier characteristics. However, its thermal stability is so poor that melt molding is possible only when it is copolymerized with vinyl chloride or has plasticizers incorporated therein. Unfortunately, PVDC copolymer, which is capable of melt molding, is usually inferior to EVOH in gas barrier properties; therefore, in applications where good gas barrier properties are required, PVDC without copolymerization or a plasticizer is used in the form of coating on films by emulsion coating or solution coating. Coating is limited in thickness of the barrier layers and hence cannot be applied in applications where good gas barrier properties are required.
Another polymer known for good gas barrier properties is PAN homopolymer. Unfortunately, it possesses poor melt moldability, because of its very high melting point. On the other hand, PAN copolymer, which is capable of melt molding, is usually inferior to EVOH in gas barrier properties.
All of the above-mentioned three polymers, EVOH, PVDC, and PAN, suffer the disadvantage of deteriorating clarity (becoming turbid) in proportion to thickness. In addition, PVDC and PAN yellow upon exposure to light and heat at the time of melt molding.
It is known that these disadvantages can be alleviated by copolymerization, which, unfortunately, aggravates gas barrier properties. Therefore, what still remains unrealized is a desired gas barrier material, which exhibits good barrier properties, even under the conditions of high humidity, and which also exhibits superior clarity and melt moldability.
Polymers are also known, typified by polyallyl alcohol and polymethallyl alcohol, which are composed of repeating units of allyl alcohol. These polymers are produced by polymerization as disclosed in U.S. Pat. Nos. 2,455,722, 2,467,105, 3,285,897, 3,666,740 (Japanese Patent Publication No. 40308/1972), and 4125694, and U.K. Patent No. 854207.
Some of these disclosures mention the application of polyallyl alcohol, but mention nothing about the fact that it has extremely good gas barrier properties. For example, U.S. Pat. No. 4,125,694 merely enumerates such applications as "coating agents, adhesives, impregnants, molding powders, paints, varnishes, laminates, fillers, in dispersions, and as intermediates in resin production", but mentions nothing about applications which require a high degree of gas barrier properties. In addition, it neither mentions nor suggests the fact that the object of the present invention is not achieved by polyallyl alcohol (as a typical allyl alcohol polymer), but is achieved only in the case where R.sup.1 in formula (1) infra, is not hydrogen, but rather is a specific alkyl group.
A polymer composed of vinyl alcohol monomer or allyl alcohol monomer is disclosed in Japanese Published Unsearched Application No. 508065/1996 (WO 95/12624), which is used as a packaging material, which exhibits an oxygen permeability lower than a certain value under conditions of high humidity. It also mentions that the allyl alcohol polymer should essentially be one which has a hydrogen atom as R.sup.1 in formula (1) of the present application: however, it discloses nothing about a polymer which has a specific alkyl group in lieu of a hydrogen atom as R'. It further mentions that the polymer composed of the vinyl alcohol units is preferred to a polymer composed of allyl alcohol units. In fact, it merely illustrates an example in which the object of improving oxygen barrier properties under conditions of high humidity is achieved by partly acrylating the hydroxyl groups of EVOH with aromatic carboxylic acid. It is also entirely different from the present invention in the way the problems are addressed.
It is also noted that literature, such as Polymer Bulletin, 3(10), pp. 521-528, 1980, reports isotactic or syndiotactic polymethallyl alcohol. However, the reference neither mentions nor suggests whether it even has gas barrier properties.
As mentioned above, among conventional gas barrier materials, EVOH is superior in having both gas barrier properties and melt moldability. There still is a demand for further improvement in gas barrier properties of polymers in order to make it possible to reduce the thickness of the gas barrier layer, thereby reducing production costs, and to lessen the effect of thickness fluctuations. There is also a demand for expanding areas of application and product variety by improving gas barrier properties under conditions of high humidity, by improving moisture barrier properties, and by improving clarity.